This invention relates to a novel catalytic process for converting olefins to heavier hydrocarbons.
Shape-selective oligomerization, as it applies to the conversion of C.sub.2 -C.sub.10 olefins over aluminosilicate ZSM-5, is known to produce higher olefins up to C.sub.30 and higher. As reported by Garwood in Preprints, Div. Petrol. Chem., ACS, 27(2), 563(1983), reaction conditions favoring higher molecular weight product are low temperature (200.degree.-260.degree. C.), high pressure (300-1500 psig), and long contact time (0.5-1 WHSV). The reaction under these conditions proceeds through the acid-catalyzed steps of (1) oligomerization, (2) isomerization-cracking to a mixture of intermediate carbon number olefins, and (3) copolymerization to give a continuous boiling product containing all carbon numbers. The channel systems of ZSM-5 type zeolite catalysts impose shape-selective constraints on the configuration of the large molecules, accounting for significant differences with other catalysts.
The following model reaction path for propene is set forth for purposes of explanation, and it should be taken as a theoretical path, as the process is presently understood by workers in the field. ##STR1## The desired oligomerization-polymerization products are substantially linear aliphatic hydrocarbons. The ZSM-5 catalytic path for propene feed provides a long chain with one methyl substituent per 4-5 carbon atoms in the straight chain. There are four distinct reactions occurring. First, propene will oligomerize to distinct C.sub.6, C.sub.9, C.sub.12, etc. oligomers. These then isomerize and recrack, forming a range of light olefins. These intermediates then repolymerize to an equilibrium (or pseudoequilibrium) distribution of heavier iso-olefin. As a result of having both forward (polymerization) and reverse (cracking), a continuous molecular weight distribution will occur in the product which can be independent of the carbon number of the starting olefin. For example, Garwood has previously shown, at constant temperature and pressure, virtually identical product distribution for feedstocks of ethene (C.sub.2.sup.=), propene (C.sub.3.sup.=), pentene (C.sub.5.sup.=), hexene (C.sub.6.sup.=), and decene (C.sub.10.sup.=). Structurally the final product is influenced by the pore structure of the catalyst. For low carbon number products (i.e., C.sub.4, C.sub.5) isomer distribution is approximately at equilibrium. For the higher carbon numbers, the structure is primarily a methyl-branched straight olefinic chain, with the maximum cross section of the chain limited by the dimension of the largest ZSM-5 pore. At conditions chosen to maximize heavy distillate range products (C.sub.10.sup.+) the raw aliphatic product is essentially mono-olefinic, with 10% or less of the double bond in the alpha position. Overall branching is not extensive, with most branches being methyl.